The invention relates to comfort and utility heating processes.
Comfort and utility heating processes are widely dependent on burning fuel gases with air in a variety of furnaces. Prominent fuel gases include hydrocarbon gases, such as methane or propane, mixtures of carbon monoxide and hydrogen, and blends of carbon monoxide, hydrogen, and hydrocarbon gases. Frequently, these gases contain noncombustible species such as nitrogen and water.
Whatever the fuel used, it is well known that conventional furnaces rarely utilize all of the potentially useful combustion heat. In broad terms, this inefficiency results because combustion gases are conducted away from the combustion zone through heat exchange arrangements that extract only a portion of the available thermal energy of the combustion gases so that the combustion gases remain at sufficiently high temperatures to facilitate effective convective ejection of the combustion gases through stacks and the like. It is also well known that an increased portion of the thermal energy of the combustion gases can be extracted when forced drafts are provided to remove and dispose of the combustion gases. However, even forced draft systems, until fairly recently, have rarely deliberately cooled the combustion gases below the so-called dew point (that temperature at which the concentration of water vapor in the combustion gases is high enough to reach or exceed saturation).
Combustion of fuel gas with air results in the formation of water vapor and carbon dioxide as principal combustion products. In addition, depending on the air-to-fuel gas feed ratio, small amounts of carbon monoxide are formed, and, depending on combustion temperature, oxides of nitrogen (designated NOx) are formed. Because air supplied to the combustion zone includes about four volumes of nitrogen for each volume of oxygen, the combustion gases inevitably contain a large fraction of nitrogen in addition to the combustion products. Along with oxygen and nitrogen, the supply air always includes water vapor and small amounts of other gases (argon, CO2, transient hydrocarbons, and, occasionally, sulfur or halogen-bearing volatiles). The water vapor in the supply air slightly increases the moisture of the combustion gases. It is also noteworthy that the ratio of water to carbon dioxide in the combustion gases is quite dependent on the fuel gas being burned. Propane, with a higher carbon/hydrogen ratio (3:8) than methane (1:4), yields less water vapor; on the other hand, some natural or manufactured gases inherently carry quantities of water.
Dew point is not the same for all fuel gas combustion processes. Besides the factors cited above, dew point is influenced by the oxygen concentration used in converting the fuel to carbon dioxide and water. In some industrial processes, for example, supply air is occasionally enriched with raw oxygen to increase the flame temperature. In these processes, because the fuel gas contains less nitrogen, it contains a higher partial pressure of water vapor. In other processes, use of excess supply air results in a higher quantity of nitrogen and a lower partial pressure of water vapor in the combustion gas. For practical purposes, however, in the uses to which the present invention primarily applies, it is reasonable to expect a dew point of within a few degrees of 65.degree. C. (150.degree. F.).
Traditional furnaces use an indirect-fired heating process to transfer combustion heat across a barrier and into a fluid such as air or water. In most cases, these furnaces transfer as little as 60% of the combustion heat into the heated fluid, the balance being retained in the combustion gases to assure their efficient disposal by thermal convection.
This waste of heat prompted several developments. One, the so-called direct-fired process, directly blends air to be heated with combustion gases without using an intervening barrier. While the direct-fired process completely eliminates stack heat losses, it is unsuitable for heating a stream of recirculating ambient air because of the potential for noxious gas buildup. As a result, outdoor air, which is invariably colder than a space to be heated, is almost always used for direct-fired heating. Thus, while the direct-fired process is efficient in the sense of using all of the thermal energy of the fuel, it is inefficient from the point of view of conserving heat in the space to be heated. The direct-fired process is most suitable for use as a secondary heat source in a space that has a primary heat source and suffers air losses from time to time. For example, the direct-fired process is suitable for use in a warehouse that has frequent opening of doors and resulting air loss to the outside. When the direct-fired process is used as the primary heat source, continuous leakage of air to the outdoors must balance the flame-heated air brought in by the direct-fired heater to keep the fraction of noxious gases tolerably low.
Indirect-fired units operating under normal conditions emit approximately 50 to 170 ppm of CO (carbon monoxide), a maximum of 110 ppm of NOx, and 80,000 to 120,000 ppm of CO2, all of which is vented to the atmosphere. Direct-fired units operating under normal conditions emit approximately 3 to 5 ppm of CO, 3 to 8 ppm of NOx, and a maximum of 1700 ppm of CO2, all of which is diluted by outside air as it enters the building.
Another approach to recovering heat from the combustion gases is through use of so-called "high efficiency" indirect-fired furnaces. These furnaces use two heat exchange zones: a primary zone in which heat is exchanged through a barrier from the hot combustion gases to preheated ambient air, and a secondary zone in which the combustion gases are further cooled by preheating the ambient air. Unlike conventional indirect-fired furnaces, combustion gases leaving the primary zone are not removed by thermal convection. Instead, a suction fan draws the combustion gases through the secondary zone, where they are cooled by a counter flow of incoming ambient air. Thus, heat exchange in the secondary zone preheats the incoming ambient air before it enters the primary zone.
The two-zone process has the desired effect of recovering substantially all of the sensible heat in the combustion gases that, in a conventional indirect-fired furnace, would be lost to the outside. As noted, a consequence of the two-zone process is that the combustion gas density increases to a point at which convective ejection is no longer feasible. As a result, the combustion gases must be withdrawn and discharged from the secondary zone by means of a positive air conveyance device such as a blower or fan. Due to cooling, the combustion gases are also reduced in volume. These two effects (cooler temperature and lower volume) make it possible to discharge the combustion gases through smaller ducts that are made from materials, such as polymers, that would be unsuitable for discharging the higher temperature combustion gases of conventional furnaces.
A recognized drawback of the two-zone process is that cooling of the combustion gases to near ambient temperature in the secondary heat-exchange zone inevitably results in the temperature of the combustion gases dropping below their dew point. This causes water vapor to condense as droplets or films on the exhaust-side surfaces of the secondary heat exchanger. While this has the desirable effect of recovering the latent heat of evaporation, impurities in the fuel gas and supply air can cause the resulting water condensate to be highly acidic. As noted above, the combustion gases contain traces of carbon monoxide and nitrogen oxides. Sometimes, the combustion gases also contain small amounts of sulfur oxides and/or hydrochloric acid vapor that is generated by decomposition of chlorine-bearing volatiles carried into the flame zone as contaminants of the fuel gas or supply air. Because all gases are capable of dissolving to one extent or another in water, these corrosive impurities are absorbed in the condensed water vapor. Absorption is further enhanced by the near ambient temperature of the combustion gases in the secondary zone. The result is a highly corrosive liquid that, in addition to jeopardizing the useful life of materials (even most grades of stainless steel) used to form the secondary heat exchanger, is environmentally offensive and may be illegal to discharge into municipal sewage systems without neutralization.
Another approach to recovering heat from the combustion gas is to scavenge the heat from the stack. For example, Astle, U.S. Pat. No. 4,754,806, shows a device that is very effective at removing stack heat, but is intended to work downstream of the primary heat exchanger of the conventional furnace.
Neither direct-fired furnaces, "high efficiency" indirect-fired furnaces, nor stack heat scavenging systems have any effect on the primary heat transfer stage taking place in the conventional furnace fire-box. Instead, they are designed to reduce the combustion heat losses occasioned by the convective ejection through stacks of combustion gases at temperatures several hundred degrees above ambient.
To solve many of the problems of the previously discussed approaches, the present inventor has provided a system in which a movable heat sink is used as the primary heat exchanger. This system, which is described in Astle, U.S. Pat. No. 5,005,556, the contents of which are incorporated by reference, permits recovery of the heat lost in a conventional single-zone furnace without contaminating the ambient air with combustion gas, as occurs in direct-fired systems, and without incurring the problems of corrosive liquids inherent in two-zone indirect-fired systems.